In addition to Western pundits commenting nightly that such an occurrence is an international disaster, the world seems to be taking up sides over the possibility that any other former Russian territory may also choose to re-unite with Russia, and sabres are already rattling all round.
We tend to forget that, although the world map has looked more or less the same since the end of World War II, it has been the norm, throughout history, for large parcels of property to change hands fairly often. Boundaries move. Countries become larger, smaller, or disappear altogether. Large empires are created, swallowing up smaller countries, sometimes lasting for 200 years or more, then inexorably breaking up into smaller remnants.